This is reason why I am atheist .
1) God is a human creation.
All the hundreds of religions/sects and their multiple absolute contradictions seem to be plenty evidence that all gods are human creations.
The same conclusion can be based on the known influences of ancient myths and religions on the current main religions [the flood, the virgin birth, the resurrection after 3 days, Christmas day, Sunday (day of the Sun, the roman god Sol Invictus) as the holyday and not the Sabbath, etc.].
Gods are just one of the illusions mankind uses in order to be able to deal with the conscience of the inevitability of death. Humans created a god and an afterlife mainly because they feel anguish about dying.
Even in the religions that claim to worship the same god, the contradictions are overwhelming.
As you know, both Christians and Muslims say they worship the Torah's god, Yahweh. Islam says Jesus was an important prophet, but not the son of god. And Christians simple reject that Muhammad was a prophet. But the Qur'an says that his god is the god that sent Abraham, Moses and Jesus.
But Yahweh initially was just a god in the middle of others. Most Jews, even during David times (about 1000 BC) and after, kept praying to other gods of the Canaanites (Semitic people comprising the Phoenicians, the Jews and some other peoples of the Levant).
There is controversy, but Yahweh has been identified with EL, the supreme god of the Canaanites, that had one or two wives and an extensive number of sons (see
http://en.wikipedia.org/wiki/El_(deity)#Hebrew_Bible). Or, initially, with one of his sons: sometimes, Baal (the confusion was easy, because Baal means Lord; clearly, later, the Torah fights this identification, by ridiculing Baal), sometimes Hadad, sometimes a different son.
In some of the Jewish holy books, we can still find several traces of this evolution, with references to a council of the gods presided by EL/Yahweh (Psalm 82:1 and 6; 1 Kings 22:19) or to different gods (Deuteronomy 32:8–9) (see, a summary in
http://en.wikipedia.org/wiki/Divine_Council#Hebrew).
Well, the Greeks were influenced by the Phoenicians and copied their gods, with different names. El was Uranus, the father of all gods (or sometimes Cronus, since some mythology says El was not the original god, but rather Elioun), that was deposed by his son, Cronus. Cronus was deposed by Zeus. The Romans used the same Gods (Caelus as Uranus; Saturn as Cronus and Jupiter as Zeus).
So, are the believers on the three main religions praying to Uranus (Caelus) or even to Cronus (Saturn)?
But, even if they are considered the same god, just compare the vengeful and jealous god of the Torah with the loving and forgiven god invented by Jesus.
The contradictions are so big between them that some scholars (like Marcion of Sinope:
https://en.wikipedia.org/wiki/Marcionism) and christian sects (like the Gnostics:
https://en.wikipedia.org/wiki/Gnosticism#Dualism_and_monism) even defended that Yahweh, the Torah's god, was a different god or even the devil.
Some of the most fracturing religious issues, like the so-called divine nature of Jesus, or its degree, divided drastically Christians and were finally settled by bishops on majority voting, under pressure from Constantine to reach an agreement.
If Constantine, as roman emperor, was considered divine, how could Jesus be less than him? Of course, we can't find any evidence on the Gospels for that (not even on John's Gospel), but they couldn't care less for that detail.
Most Christian churches defend the Trinity, that the father, the son (Jesus) and the holy ghost are not exactly one and the same, but are part of god. But these churches argue that this is perfectly compatible with a monotheism.
Basically, Jesus on the Olive Garden and on the Cross wasn't exactly talking with him self, but something similar (if he was already complaining on the Garden, I imagine the family discussion when he arrived "home").
Ancient Greeks could argue that they also had a father, Uranus/Cronus/Zeus, and their sons and parents, all part of a divine family. That the difference was of grade, and not nature, and so that they too were basically monotheists in this flexible sense, because they too had a supreme god, he just had a bigger family.
But what all these contradictions, but also influences and slow evolution, point out is that gods are a human creation.
With all these different gods and interpretations, are all the believers on different religions or sects lying or mistaken, but you?